realtek: XGS1210-12: convert RTL8226 PHYs to 2500base-x

We reached the point of no return. Upstream has gained the final
bits for the RTL8226 PHYs. That means.

- RTL8226 MAC side behaves like RTL8221(B)
- It's serdes no longer uses proprietary HSGMII (2.5G SGMII)
- Instead it dynamically switches between SGMII and 2500base-x

This (partly) solves one of the central henn/egg problems of the
Realtek target. To change the MAC/PHY interface mode both sides
need to have all bits in place to do so. But where to start if
so much needs to be done?

Now the PHY side has created facts and it mitigates a lot of
problems. All downstream HSGMII patches and coding can be dropped
in the future.

For now only adapt the only DTS that still maps PHYs to HSGMII.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/19843
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

target/linux/realtek/dts/rtl9302_zyxel_xgs1210-12.dts

@@ -290,14 +290,14 @@
 		port@24 {
 			reg = <24>;
 			label = "lan9";
-			phy-mode = "hsgmii";
+			phy-mode = "2500base-x";
 			phy-handle = <&phy24>;
 			led-set = <1>;
 		};
 		port@25 {
 			reg = <25>;
 			label = "lan10";
-			phy-mode = "hsgmii";
+			phy-mode = "2500base-x";
 			phy-handle = <&phy25>;
 			led-set = <1>;
 		};